1. Field of the Invention
This invention relates to a hydrostatic drive system with a user which is in the form of a differential cylinder and is connected in a closed circuit to a high-pressure pump. A piston-side compression chamber of the differential cylinder is in communication by means of a first power fluid line and a piston-rod-side compression chamber of the differential cylinder is in communication by means of a second power fluid line with the high-pressure pump. The closed circuit comprises a protection device which is formed by a first protection valve device that comprises a pressure relief valve and an anti-cavitation valve which is associated with the first power fluid line, and a second protection valve device which comprises a pressure relief valve and an anti-cavitation valve which is associated with the second power fluid line, which protection valve devices are connected with a low-pressure system.
2. Description of Related Art
For users that are operated in a closed circuit, a protection device is necessary which makes it possible to protect the user against excess pressure and cavitation. The protection device is thereby formed by protection valve devices which are associated with the respective power fluid lines, each of which protection valve devices has a pressure relief valve and an anti-cavitation valve that opens toward the associated power fluid line. The pressure relief valves protect the corresponding power fluid lines against excess pressure. The anti-cavitation valves perform a recharging function to prevent a cavitation in the associated power fluid line by feeding power fluid from the low-pressure system into the corresponding power fluid line.
When there is a movement of the piston, users in the form of differential cylinders have different volumes in the piston-side compression chamber and in the piston-rod-side compression chamber. If a differential cylinder of this type is connected in a closed circuit to a pump and is operated in both directions of movement, it must be guaranteed that when there is a movement of the piston of the differential cylinder, the differential volumetric flows that result from the different volumes of the piston-side compression chamber and the piston-rod-side compression chamber are equalized in the closed circuit.
EP 1 588 057 B1 describes a generic drive system in which, to equalize the differential volumetric flows of the differential cylinder operated in the closed circuit, in addition to a protection device that includes pressure relief and anti-cavitation valves, there are additional pilot-operated non-return valves in the closed circuit which connect the corresponding power fluid line of the closed circuit with a low-pressure system. The non-return valves are each actuated into an open position by the pressure in the opposite power fluid line. The non-return valve that opens toward the low-pressure-side power fluid line is, therefore, actuated into the open position by the high pressure of the closed circuit that is present in the high-pressure-side power fluid line to equalize the differential volumetric flow by feeding an additional volumetric flow from the low-pressure system into the corresponding low-pressure-side power fluid line of the closed circuit or by allowing the excess volumetric flow to flow out of the corresponding low-pressure-side power fluid line of the closed circuit into the low-pressure system.
However, additional pilot-operated non-return valves of this type increase the cost of construction and the space occupied by the drive system because additional valves are necessary for the equalization of the differential volumetric flows and in particular for the removal of the excess volumetric flow from the closed circuit.